The present invention relates to a process and an apparatus for producing oxidic nanocrystals.
Oxidic nanocrystals, in particular doped oxidic nanocrystals, such as for example europium-activated yttrium oxide (Y2O3:Eu) have been paid increasing attention in recent years for use as phosphors for illumination equipment (U.S. Pat. No. 5,455,489). For this reason, a very wide variety of oxidic nanocrystals have been tested for suitability for use as a phosphor. Consequently, corresponding processes and apparatus for producing nanocrystals of this type have also been proposed.
For example, to produce Y2O3:Eu, gas condensation techniques are known, cf. for example D. K. Williams et al., J.Phys.Chem.B 102(1998), p.916. However, a drawback of these known processes is that the resultant oxidic nanocrystals are of highly amorphous structure and, in addition, have a monoclinic crystal structure. However, for the oxidic nanocrystals to be used as phosphors it is desirable for them to be in the cubic equilibrium phase, since this is when the light efficiency is highest.
To achieve this, hitherto the amorphous or monoclinic phase has been transformed into the cubic phase by an additional heat treatment at approximately 900xc2x0 C. However, this entails considerable grain growth, and consequently the cubic phase is only present as a single phase at ultimate particle sizes of over about 50 nm. This in turn, on the one hand, means a high outlay in terms of energy and time for the production of nanocrystals of this type. On the other hand, the relatively large ultimate particle size considerably limits the possible applications of these oxidic nanocrystals which are produced using processes according to the prior art. Hitherto, it has not been possible to produce phosphors with small ultimate particle sizes, in particular of up to 20 nm, which have a high level of efficiency. The term ultimate particle size is understood to mean the grain size of the phosphor following a heat treatment which may lead to partial agglomeration. The agglomeration can be classified as low if there is as yet no sintering of the grains at the boundary surfaces, although grains may already be arranged next to one another in rows. By contrast, the term initial particle size is used if the original grain size prior to a heat treatment is what is meant.
The object of the present invention is to provide a process for producing oxidic nanocrystals which ensures that suitable cubic nanocrystals of small ultimate particle sizes can be produced easily without an additional heat treatment.
A further object of the present invention is to provide an apparatus for producing oxidic nanocrystals which overcomes the drawbacks of the prior art cited in the introduction.
These objects are achieved by a process and an apparatus in accordance with the features of the independent claims.
Advantageous configurations are described in the subclaims.
A process according to the invention for producing oxidic nanocrystals comprises the following steps:
a) Introducing host lattice ions as metalorganic complexes or volatile inorganic compounds into at least a first vaporizer,
b) Converting the complexes or compounds into the gas phase and
c) Transporting the resultant gas phase, with the aid of one or more carrier gases and/or carrier gas mixtures, into at least one heating zone of at least one reactor, and supplying one or more reaction gases and/or reaction gas mixtures (into the heating zone), and
d) Breaking down the complexes or compounds in the heating zone of the reactor at a pressure of between 1-1000 mbar, the oxidic nanocrystals being formed immediately, and
e) Collecting the oxidic nanocrystals at at least one adsorption trap.
The term vaporizer in this context means a vaporization system of which the essential components are vessel and associated heating means.
With this process according to the invention, it is possible to produce oxidic nanocrystals which are present in a thermodynamic equilibrium phase and have a mean initial particle size of at most 100 nm, preferably of 1 to 20 nm, in particular an initial particle size in the region of 10 nm, and a cubic crystal structure. They may in particular be relatively unagglomerated, preferably with a mean ultimate particle size of up to 20 nm. The cubic equilibrium phase of the oxide is reached even during the production process. The cubic equilibrium phase is particularly important for phosphors with a high quantum efficiency which are based on oxides of this type, such as for example yttrium oxide (Y2O3), gadolinium oxide (Gd2O3) or corresponding garnets, such as cerium-activated yttrium aluminum garnet, YAG:Ce or cerium-activated yttrium gadolinium aluminum garnet, (YGd)AG:Ce.
By suitably conducting the reaction, it is possible in particular to produce phosphors with a narrow size distribution. A fluctuation range of 2 to 5 nm about the mean initial particle size is typical.
In general terms, the process is suitable in particular for oxidic or oxysulfidic nanocrystals, particularly based on the elements Y, Gd, Mg, Ca, Ba and Al and other rare-earth elements.
In an advantageous configuration of the process according to the invention, in process step a), together with the host lattice ions, doping ions are introduced into a second vaporizer as metalorganic complexes or volatile inorganic compounds and are converted into the gas phase. The resultant gas phase is transported into the heating zone of the reactor with the aid of one or more carrier gases and/or carrier gas mixtures. This makes it possible to produce not only undoped oxides but also doped, (primarily in the sense of an activation) nanocrystalline oxides. In this case, the doping is incorporated directly into the host lattice.
The undoped or doped nanocrystalline oxides are used as phosphors for illumination equipment with low or high luminances. Consideration may be given in particular to fluorescent lamps, noble gas (Xe) excimer lamps, displays and organic and inorganic LEDs. Depending on the particular requirements, the host lattice ions are elements from main groups II to VI of the Periodic System of the Elements. The doping ions are rare-earth elements such as Eu, Ce or Tb. If metalorganic complexes are used, compounds of the host lattice ions (or doping ions) with hydrogen tetramethylheptanedionate are eminently suitable for the process. Examples of suitable volatile inorganic compounds are chlorides.
In a further advantageous configuration of the process according to the invention, the first and second vaporizers are each at a temperature of approximately 30xc2x0-900xc2x0 C. The temperature in the heating zone of the reactor is approximately 600xc2x0-1600xc2x0 C. Furthermore, inert gases or gas mixtures are used as carrier gases. The noble gases or nitrogen or mixtures thereof are used in particular. Oxygen or an oxygen-containing mixture has proven to be a suitable reaction gas.
In another advantageous configuration of the process according to the invention, the flow rate of the carrier gas or the carrier gas mixtures in the system is 20 ml/min to 50 l/min. The flow rate of the reaction gas and/or the reaction gas mixtures is 50 ml/min to 100 l/min.
According to the invention, the pressure in the entire system is held at 1 to 1500 mbar.
An apparatus according to the invention for producing oxidic nanocrystals comprises at least one vaporizer, at least one reactor with at least one heating zone, at least one adsorption trap and at least one vacuum pump. With this apparatus it is possible to produce the desired oxidic nanocrystals for use as phosphors. A further, additional heat treatment outside the heating zone of the reactor in order to obtain the desired cubic crystal form of the nanocrystals is no longer required, unlike in the prior art.
In an advantageous configuration of the invention, the vaporizers have vaporizer heating means which are designed as oil baths. The temperature of the oil baths is in each case approximately 140xc2x0 to 180xc2x0 C. The reactor is advantageously designed as a flow reactor.
In a further advantageous configuration of the apparatus according to the invention, in each case one heatable tube is arranged between the vaporizer(s) and the reactor. According to the invention, this prevents the metalorganic complexes or inorganic compounds which have been converted into the gas phase from condensing early.
In a further advantageous configuration of the apparatus according to the invention, the reactor comprises an aluminum tube and a cylindrically designed heating zone. A baffle is arranged inside the reactor at the end of the heating zone. The baffle which is arranged in this way prevents convection in the reverse direction, i.e. counter to the direction in which the gas phase flows in, so that the formation of the oxidic nanocrystals takes place in the desired areas of the heating zone of the reactor.
In a further advantageous configuration of the apparatus according to the invention, a filter is arranged between a second adsorption trap, which is connected downstream of the first adsorption trap, and the vacuum pump. The second adsorption trap and the filter prevent contamination to the vacuum pump. The adsorption traps are usually designed as cold traps.
Further details, features and advantages of the process according to the invention and the apparatus according to the invention are explained below with reference to an exemplary embodiment illustrated in the figures, in which: